Large and tunable magnetocaloric effect in gadolinium-organic framework: tuning by solvent exchange

Magnetic properties of three variants of MOF-76(Gd), {[Gd(BTC)(H₂O)]·G}_n (BTC = benzene-1,3,5-tricarboxylate, G = guest molecules) were investigated by static susceptibility, isothermal magnetization and specific heat capacity measurements. In the study we used as synthesized MOF-76(Gd)-DMF (1) (G = DMF = dimethylformamide), containing DMF molecules in the cavity system, compound MOF-76(Gd) (2), activated complex without solvents in the cavities and water exchanged sample MOF-76(Gd)-H₂O (3). A pronounced change in the magnetic entropy was found near the critical temperature for all three compounds. It was shown, that magnetic entropy change depends on the solvatation of the MOF. The highest value entropy change, ΔS_Mpk(T) was observed for compound 2 (ΔS_Mpk(T) = 42 J kg^-1 K^-1 at 1.8 K for ΔH = 5 T). The ΔS_Mpk(T) for the compounds 1, 2 and 3 reached 81.8, 88.4 and 100% of the theoretical values, respectively. This suggests that in compound 3 Gd³⁺···Gd³⁺ antiferromagnetic interactions are decoupled gradually, and higher fields promote a larger decoupling between the individual spin centers. The observed entropy changes of compounds were comparable with other magnetic refrigerants proposed for low-temperature applications. To study the magnetothermal effect of 2 (the sample with largest -ΔS_Mpk), the temperature-dependent heat capacities (C) at different fields were measured. The value of magnetic entropy S obtained from heat capacities (39.5 J kg^-1 K^-1 at 1.8 K for an applied magnetic field change of 5 T) was in good agreement with that derived from the magnetization data (42 J kg^-1 K^-1 at 1.8 K).